1. Field of the Invention
The invention is related to the field of ionic probes.
2. Statement of the Problem
A measure of the ionic concentration of a fluid is desirable in many situations, including testing of fluids in manufacturing settings, for pharmaceutical production, food processing and/or food quality, water quality testing, etc. Measurement of an ionic concentration or activity can indicate completion of a reaction, indicate fractions of components, etc.
One measure can comprise a measure of a pH concentration, which comprises a measure of acidity of the fluid being tested. The pH measurement can indicate the acidic or basic condition or concentration of the fluid.
A pH measurement comprises a measurement of hydrogen ions in a solution, expressed as a logarithmic number between about zero and fourteen (sometimes extending into negative numbers for exceedingly acidic solutions). On the pH scale, a very acidic solution has a low pH value, such as zero or one, corresponding to a large concentration of hydrogen ions (H+). In contrast, a very basic solution has a high pH value, corresponding to a very small number of hydrogen ions (or to a correspondingly large number of OH− ions). A neutral solution, such as substantially pure water, has a pH value of about seven.
FIG. 1 shows a portion of a prior art pH meter probe including an active electrode and a reference electrode. The active electrode consists of a glass tube with an ion sensitive glass bulb at one end. The tube contains an electrolyte and an electrode. The glass on the exterior of the ion sensitive bulb exchanges ions with the fluid to be tested. This produces a charge in a hydrated layer on the outside of the bulb. The internal electrolyte interacts with the ion sensitive glass and reflects the potential developed by the ions at the outside of the glass.
The reference electrode is often included in a separate chamber and solution, and is also in ionic communication with the fluid being tested. A voltage potential between the two electrodes is thereby formed, similar to a battery. The voltage potential that is developed between the electrodes is directly related to the ion concentration of the solution. The reference electrode provides a stable potential against which the measuring electrode can be compared. The voltage potential can be processed according to a table, formula, or other algorithm to arrive at an ionic concentration measurement, such as a pH value, for example.
An ionic circuit is formed between the active electrode and a ground electrode, creating a measurable voltage potential. The reference potential is a known, substantially constant amount against which the process voltage (i.e., a voltage measurement) can be compared and interpreted by the prior art pH meter. The voltage potential between the active electrode and the reference electrode can be processed to determine an ionic concentration in the external test fluid.
The accuracy of ionic and/or pH measurements can be affected by various factors, including temperature and/or contaminated electrolyte solutions, for example. A common source of inaccuracy can be an improper or inaccurate reference signal generated from a reference electrode. If the reference signal is inaccurate, the resulting pH or ion measurement will be affected. Consequently, it is of great importance that a proper and accurate reference value be obtained.
The reference electrode is contained within a tube or chamber that includes an ionic bridge, such as a salt bridge, that enables ionic communication between the reference electrode and the external test fluid. However, the ionic bridge may allow some fluid exchange, enabling contamination of the internal buffer solution and possible poisoning of the internal reference electrode, and enabling contamination of the fluid to be measured.
A major problem with pH probes is in the junction between the internal fill solution of the reference electrode assembly and the external test fluid. Clogging or failure of the junction usually leads to very slow or erroneous readings. The junction can also allow the contamination of the fill solution with the measurement medium. This can degrade the reference electrode which then renders the pH probe inaccurate and it usually has to be replaced. One prior art solution has been the employment of multiple junctions and chambers between the reference electrode and the exterior medium. Another prior art solution has used flowing junctions in which a continuous supply of fill solution is fed to the reference electrode compartment and exits via a small hole or conduit. This has the advantage of preventing the contamination of the fill solution and the reference electrode but has the disadvantage of cumbersome plumbing to the electrode and the necessity to send the measurement medium to waste as it is contaminated with fill solution.
A newer approach has been to enclose both the active electrode and the reference electrode within an impermeable chamber, such as a glass chamber, for example. This is shown in U.S. Pat. No. 4,650,562 to Harman. The reference electrode 12 in Harman interfaces with the external test fluid through a pH sensitive glass bulb, similar to the structure of the active electrode 11. The external test fluid therefore cannot mingle with and contaminate the internal fill solution of the reference electrode.